Many processes require the mixing of solid particles of different materials, particularly when such particles are relatively small, e.g., of powder sizes in a range from about 1 micron to about 1 millimeter. For example, such mixtures may be required in mixing dry materials to form pills or other drug dosage forms, in mixing plastic materials such as polymeric plastic paticles for molding purposes, in mixing additives to materials, such as vitamin additives to flour in bread making processes or filler material in plastics for coloring or strengthening the plastic. Other uses will occur to those in the art.
The use of presently available mechanical mixing devices tends to provide mixtures of solid particles which are described at best as "random" mixtures. A random mixture can be described as one in which the probability that any particle is of a specified type is the same at all points in the mixture, such probability being equal to the fraction of that type of particle which is in the mix. For a random mixture, as defined, the number of particles of one type in a plurality of samples of the same size follows the binomial distribution. In many applications a random mixture, or even a mixture which is not as good as a random mixture, may be adequate. Thus, random mixtures may be adequate in cases where the smallest sample size of the mixture that is of interest contains a very large number of particles, in which cases each sample size contains the mixed components in the desired ratio within an acceptable error.
However, in many applications where, for example, the smallest sample size of interest contains only a relatively small number of particles, the variation among samples associated with a random mixture may not be acceptable. Sometimes this problem can be circumvented by reducing the sizes of the particles being mixed so as to create a larger number of particles in the smallest sample size of interest. With conventional devices a random mixture is always the best that can be achieved. A random mixture of smaller particles is better than a random mixture of larger particles. However, a problem arises when the particle size cannot be reduced further than a minimum size and a better than random mixture is still needed or is at least desired.
A "perfect" mixture can be defined as one in which each component is evenly distributed throughout the mixture so that with reference to the smallest sample of interest, the ratio of the particle components in every such sample is the same as the ratio of components in the entire mixture, so long as the sample size is greater than the individual particle sizes. In many applications in which a random mixture is not acceptable, it is desirable to provide a mixture which tends toward and approaches as best as possible a perfect mixture as so defined.